1. Field of the Invention
The disclosure relates to a liquid crystal display device, and more particularly, to a backlight unit and a liquid crystal display module including the same.
2. Discussion of the Related Art
Recently, various flat panel display (FPD) devices having advantages of thin thicknesses, light weights and low power consumption have been developed and have replaced cathode ray tubes (CRTs). The FPD devices include plasma display panels (PDPs), liquid crystal display (LCD) devices, electroluminescent display (ELD) device, and so on.
PDPs display images by colliding ultraviolet rays emitted from gases contained between upper and lower substrates, for example, xenon (Xe) and neon (Ne), with a phosphor and emitting visible rays, that is, by emission of the phosphor due to violet rays produced in the discharge of gas. LCD devices display images by controlling transmittance of light emitted from a backlight unit and passing through pixels according to an electric field, which is induced in a liquid crystal layer injected between upper and lower substrates and is changed by signal voltages of the pixels. ELD devices display images by interposing an organic light-emitting material between an anode electrode and a cathode electrode and enabling organic molecules to emit light using currents.
Among the display devices, LCD devices are widely used because the LCD devices are excellent at displaying moving images and have a high contrast ratio.
Generally, an LCD device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules. In other words, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmittance.
LCD devices are not self-luminescent and require an additional light source. By disposing a backlight unit at the rear side of a liquid crystal panel to emit light into the liquid crystal panel, discernible images can be displayed.
Backlight units are classified as edge type or direct type according to the position of the light source with respect to a display panel. In edge-type backlight units, which may be referred to as side-type, one or a pair of lamps are disposed at one side or at each of two sides of a light guide panel of a backlight unit. The edge-type backlight units have an advantage of easy manufacture. In direct-type backlight units, a plurality of lamps are disposed directly under a diffuser plate and provide light to a liquid crystal panel. The direct-type backlight units are widely used for large-sized LCD devices because of high uniformity of light.
Recently, large-sized display devices having more than 20 inches in diameter have been highly requested, and the direct-type backlight units are used for the large-sized display devices because the direct-type backlight units have high efficiency of light due to a plurality of lamps and have no limitation on sizes by considering the brightness and the contrast ratio of the device.
Meanwhile, the liquid crystal panel and the backlight unit are assembled as a module with various mechanical elements to protect them from outer impacts prevent light from leaking, and this may be referred to as a liquid crystal display module.
FIG. 1 is an exploded perspective view illustrating a liquid crystal display (LCD) module including a direct-type backlight unit according to the related art. In FIG. 1, the related art LCD module 1 includes a liquid crystal panel 10, a backlight unit 20, a support main 30, a top case 40 and a cover bottom 50.
The liquid crystal panel 10 includes upper and lower substrates (not shown) and a liquid crystal layer (not shown) between the upper and lower substrates. Thin film transistors (not shown) are formed on the lower substrate, and color filters (not shown) are formed on the upper substrate. Images are displayed according to on/off states of the thin film transistors. In addition, gate and data printed circuit boards 15 are connected to the liquid crystal panel 10 and supply scan signals and data signals to the liquid crystal panel 10, respectively.
The direct-type backlight unit 20 is disposed at a rear surface of the liquid crystal panel 10 and provides the liquid crystal panel 10 with light. The backlight unit 20 includes a plurality of lamps 24 spaced apart from each other. A reflection sheet 22 is disposed under the plurality of lamps 24. A couple of support sides 33 are combined with respective sides of each lamp 24 to support the plurality of lamps 24. A diffuser plate 25 and a plurality of optical sheets 26 are disposed over the plurality of lamps 24. The diffuser plate 25 includes a plurality of beads therein.
The liquid crystal panel 10 and the backlight unit 20 are disposed on the support main 30. The support main 30 prevents the liquid crystal panel 10 and the backlight unit 20 from moving and supports the liquid crystal panel 10 and the backlight unit 20.
The top case 40 covers edges of an upper surface of the liquid crystal panel 10 and side surfaces of the support main 30 to support and protect the edges of the liquid crystal panel 10 and the side surfaces of the support main 30.
The cover bottom 50 covers a lower surface of the support main 30 and protects lower elements of the LCD module 1. The cover bottom 50 is combined with the support main 30 and the top case 40 by a connecting means (not shown) to become a module.
FIG. 2 is a cross-sectional view expanding a part of an LCD module including a direct-type backlight unit according to the related art. FIG. 3 is a cross-sectional view illustrating a diffuser plate of a direct-type backlight unit according to the related art.
In FIG. 2, the direct-type backlight unit 20 of the related art LCD module includes a reflection sheet 22, a plurality of lamps 24, a diffuser plate 25 and a plurality of optical sheets 26. The reflection sheet 22 is disposed on a cover bottom 50. The plurality of lamps 24 are disposed over the reflection sheet 22 and spaced apart from each other with a first distance S. The diffuser plate 25 is disposed over the plurality of lamps 24 with a second distance D from an upper surface of the cover bottom 50, more particularly, from a surface of the reflection sheet 22. Referring to FIG. 3, the diffuser plate 25 has flat upper and lower surfaces and includes a plurality of beads 27, which are spherically shaped and are randomly distributed. Accordingly, when lights from the plurality of lamps 24 are incident on the diffuser plate 25, paths of the lights are changed by the beads 27, and the lights are diffused.
However, even though the diffuser plate 25 is used, there may exist a brightness difference between first regions A corresponding to the lamps 24 and second regions B corresponding to portions between adjacent lamps 24 if a distance between adjacent lamps 24 is increased more than the diffusing ability of the diffuser plate 25. The brightness difference causes mura defects on displayed images. To prevent this, the plurality of lamps 24 are arranged with the predetermined distance S, and the diffuser plate 25 is disposed over the lamps 24 with the predetermined second distance D from the reflection sheet 22, thereby diffusing lights and averaging the bright distribution. At this time, the ratio of the first distance S between adjacent lamps 24 and the second distance D between the reflection sheet 22 and the diffuser plate 25, that is, a value of the first distance S over the second distance D, which may be referred to as a distance ratio S/D, is a very important factor for preventing the mura defects due to the brightness difference.
Generally, in case that one diffuser plate 25 including the plurality of beads 27 randomly diffused therein is disposed over the lamps 24 as stated above, there is no mura defect due to the brightness difference if the diffuser plate 25 and the lamps 24 are arranged such that the distance ratio S/D equals to or is less than 1.35.
However, in the related art LCD module, when the lamps 24 and the diffuser plate 25 are arranged such that the distance ratio S/D is within a range of 1.35 to 2.75, there occur mura defects in which line-shaped dark portions are periodically shown due to the brightness difference as shown in FIG. 4. FIG. 4 is a photo showing a white image displayed in the related art LCD module including lamps and a diffuser plate that are arranged with the distance ratio S/D within a range of 1.35 to 2.75.
FIG. 5 is a view of illustrating a part of an LCD module according to another embodiment of the related art and shows a diffuser plate and lamps. FIG. 6 is a view showing status of lights passing through the diffuser plate from the lamps. Here, a distance between adjacent lamps is widened as compared with that of FIG. 2.
In FIGS. 5 and 6, to reduce manufacturing costs, the number of lamps 24 is decreased, and the first distance between adjacent lamps is increased and becomes 2S. The first distance of FIG. 5 is twice wider than the first distance S of FIG. 2. At this time, lights emitted from the lamps 24 have different paths in reaching the diffuser plate 24, and there are regions C in which the brightness is rapidly lowered due to path difference. Accordingly, this causes brightness difference, and mura defects occur because the path difference is beyond the diffusing ability of the diffuser plate 25.
Meanwhile, if a thickness or height of the cover bottom 50 is reduced to manufacture a thin device, the second distance D between the reflection sheet 22 and the diffuser plate 25 is decreased. Accordingly, the distance ratio is larger than 1.35, and thus there occur mura defects.